Sewage sludge also known as biosolids is a solid, semi-solid, or liquid residue generated during the treatment of domestic sewage in special treatment devices. Sewage sludge includes, but is not limited to, domestic septage; scum or solids removed in primary, secondary, or advanced waste water processes; and a material derived from sewage sludge. Sewage sludge does not generally include ash generated during the firing of sewage sludge in a sewage sludge incinerator or grit and screenings generated during preliminary treatment of domestic sewage sludge in the treatment device.
Domestic septage is either liquid or solid material removed from a septic tank, cesspool, portable toilet, Type III Marine sanitation device, or similar treatment device that receives only domestic sewage. Domestic septage does not generally include liquid or solid material removed from a septic tank, cesspool, or similar treatment device that receive either commercial waste water or industrial waste water, and does not generally include grease removed from a grease trap at a restaurant. Domestic sewage is generally waste and waste water from humans or household operations that is discharged to or otherwise enters a treatment device.
Sewage sludge treatment has been known to include three basic processes: preliminary treatment, primary settlement, and secondary treatment. Preliminary treatment involves grit removal and screening to remove larger material such as rags, towels, etc. In primary settlement, the sewage sludge flows through large tanks where smaller organic material is allowed to drop out. During the secondary treatment stage, the mixed liquor is aerated to aid bacteria in breaking down its mass, after which the resulting secondary sludge is allowed to settle. The sludge produced by the primary settlement process and the secondary oxidation process is combined to form the untreated sludge often known as raw sludge. This raw sludge can then be fed directly to a dryer or to a digester for further digestion by anaerobic or aerobic bacteria after which it can be fed to a dryer. Prior to drying the raw sludge is dewatered. Free water is removed by mechanical means such as a belt press or centrifuge to achieve around 30% dry solids. The material produced is referred to as sludge cake, which may be stored for later transportation to another site for drying. The sludge now passes to a dryer for removal of the remaining water to produce the finished product, normally 90-95% dry solids.
Drying can be accomplished either by convection drying when hot gas/air is blown through or over the sewage sludge or by conduction drying whereby the sewage sludge is brought into contact with a heated surface. In the case of convection drying, the gas (air) flowing through the dryer can be heated directly or indirectly. With direct heating, the hot waste gas (oxygen depleted) from a combustion chamber is fed into the dryer, while with indirect heating, air is heated via a heat exchanger. With conduction drying, heat is usually provided by either steam or from a hot oil system. The dryer can have various combinations of heated jackets and hollow paddles/discs through which the heating medium flows. There are various types of known thermal dryers such as: (1) horizontal drum dryers (e.g., rotary dryers, paddle dryers, and thin film dryers); (2) vertical tray dryer-pelletizers; (3) conveyor belt dryers; and (4) fluidized bed dryers.
Sewage sludge is known to contain pathogens which are disease causing organisms such as certain bacteria, viruses, and parasites. These pathogens when present in the dried sewage sludge may be dangerous to humans and other animal life. Sewage sludge often attracts rodents, flies, mosquitoes, or other organisms capable of transporting infectious agents.
In 1993, the U.S. Environmental Protection Agency (EPA) promulgated rules to ensure public safety in the treatment and handling of sewage sludge. In particular, the EPA issued rules known as “Part 053 Rule” and described in EPA publications EPA/625/R-92/013 (December 1992) and entitled “Control of Pathogens and Vector Attractions in Sewage Sludge”; EPA/832-B-92-005 entitled “Domestic Septage Regulatory Guidance: A Guide to the EPA 503 Rule,” and EPA/832/R-93/003 (September 1994) entitled “A Plain English Guide to the EPA Part 503 Biosolids Rule.” Particularly, Chapter 5 of EPA/832/R-93/003 entitled “Pathogen and Vector Attraction Reduction Requirements” addresses treatment of biosolids to address pathogens.
Accordingly, despite the known drying equipment and methods for treatment of pathogen containing sewage sludge, there remains the need for improvements, for example, an apparatus and method which enable the processing of pathogen containing sewage sludge under the EPA Rules and guidelines.